It was shown in earlier work that cis- and trans-isomers of pilocarpine and pilocarpidine differ greatly in quantitative teratogenicity, but that with all of them nicotinamide and riboflavin have protective effects. It was found that two analogs of nicotinamide differ greatly in qualitative teratogenic effect, but that supplements of nicotinamide render both of them entirely harmless. Current work with hydroxylated pyridines shows that substitution in the 3-position of the benzene ring leads to much higher teratogenicity than that in either the 2 or 4-positions; that minor changes in substitution can cause a difference in gradient of defectiveness; that the addition to the 3- hydroxypyridine molecule of an OH-group in the 2-position, though the latter by itself is rather innocuous, not only greatly raises teratogenicity quantitatively, but causes an entirely new syndrome of defects. On the basis of these observations the project proposes to look for additional evidence which would systematically relate the position of substitutions in the benzene ring to extent and type of teratogenicity, which would search for histological and radiochemical evidence implicating specific cellular acceptors for the deviations from normal development and would search for metabolic supplements acting as antiteratogens of the major substituted and teratogenic pyridines.